The invention relates to a magneto-optical printing head for generating a picture in the form of at least one line of light spots. The head includes a light source, an imaging optical system and, between that source and imaging optics, a magneto-optical light-switching mask with light-switching elements. By means of head pulses selectively generated from thin-film resistors applied to the light-switching elements and by means of a magnetic field applied over a coil, these elements can be thermomagnetically switched between two possible states with the direction of magnetisation either parallel or antiparallel to the surface normals.
The invention further relates to a method of generating a grey-scale image on a recording substrate using such a head and to the use of the head in an optical printer.
A magneto-optical light switching mask suitable for these purposes is known from DE-PS No. 26 06 596, with the aid of which an electronically controllable high-resolution pattern of light spots can be generated.
From DE-OS No. 28 12 206 a magneto-optical printing head is known which is built up with the aid of a light-switching mask in accordance with DE-PS No. 26 06 596 and which is suitable for line-by-line illumination in an optical line printer.
The basic principle of the known light-switching mask resides in the fact that linearly polarised light upon passing through a thin ferromagnetic and magneto-optical garnet film undergoes a rotation of the plane of polarisation (Faraday rotation). This rotation changes sign when the direction of magnetisation in the film is reversed. In a polarising lens system, formed from a polarisation foil at the light entrance side in front of the mask and an analyser foil at the light exit side behind the mask, the rotation of the plane of polarisation brings about for both states of the direction of magnetisation a reversal of brightness (from light to dark). The thin magneto-optical garnet film is structured in the form of islands on a substrate. It possesses a spontaneous magnetisation which is constantly perpendicular to the surface of the film and hence either parallel or anti-parallel to the surface normals. Switching from one state to another takes place thermomagnetically. To this end a thin-film resistor is deposited by vapour-deposition at a corner of each island, to which a current pulse (heat pulse) is applied. The resultant ohmic heat raises the local temperature, thereby reducing the stability of the magnetisation direction. By means of an external magnetic field, generated for example with a coil, the magnetisation can then be switched into the reverse direction. After cooling within microseconds the new magnetisation direction is again so stable that the renewed application of an external magnetic field produces no further effect. The switching operation lasts typically 20 .mu.s.
For optical printing, switching elements integrated on a substrate are fabricated in mask technology and arranged side by side in a line so that a pattern in the form of a line of light spots can be generated in transmission. For all switching elements of a line the switching magnetic field is applied in common by a coil while heating resistors are controlled individually for the switching elements via integrated switching circuits connected to them.
In optical line printers the individual switching elements are then switched either light or dark for the duration of a line illumination, a switching cycle taking place approximately as follows. An element to be switched to the bright state is at the beginning of an illumination cycle of duration T.sub.max switched with a heat pulse I.sub.n and a positive magnetic pulse H.sup.+ into the bright state, and at the end of the illumination cycle is switched with a further heat pulse I.sub.h and a negative magnetic field pulse H.sup.- into the dark state. This procedure takes place in parallel for all switching elements of the line that are to be switched into the bright state.
In the known magneto-optical printing head a light-switching mask is arranged in front of a linear light source, such as a fluorescent lamp, a tubular halogen lamp or of a combination of a point light source (e.g. a halogen reflector lamp) and a fibre light guide cross-section converter, which conveys the light from the point light source into a slit exit. The pattern of light spots then produced in transmission behind the light-switching mask is imaged by means of an optical system onto the image plane of a recording substrate. In practice a printing head will contain a number of light-switching lines arranged side by side in modules and imaged by an optical system consisting of several parallel-arranged objectives.
A fundamental problem of such a printing head with parallel generated light spots is that in the image plane there are differences in light intensity between the individual image points, arising from disturbances in the light path due to lens vigneting, to fluctuations in the luminous output of the light source and to variations in the geometry of the light-switching elements in the light-switching mask. These errors of uniformity are normally stationary, that is to say they do not change with time; they amount typically to 20% of the maximum intensity.
Magneto-optical printing heads largely find application in electrophotographic equipment. Such equipment is currently used only for printing text or graphs, i.e. black and white information. For this application the fluctuations of intensity are acceptable since the electrophotographic processes mostly work with very "hard" characteristics, i.e. with steep exposure curves. In the future, however, a grey-scale display will also be necessary, working for example with point-density modulation or with soft recording materials and direct amplitude modulation of the illumination. In addition, other areas of application are gaining in importance in professional printing and film-illumination technologies, where in principle it is also necessary to work with a highly uniform optical illumination.